Condensation and Mould in UK Homes: Causes, Fixes, and Prevention
The short answer
- Three causes: cold surfaces, too much moisture and too little heating, so ventilate, heat and insulate together
- Ventilate first, as it is cheapest and fastest, then insulate, then heat
- At least 18°C in the rooms you use, as the NHS advises, and keep the home steadily warm
Condensation happens when warm, moist air meets cold surfaces like windows, external walls and corners. Left alone, it leads to mould, which government guidance says can harm anyone's health. The fix is not just ventilation or just insulation; it is a combination of ventilation, heating and insulation. This guide explains why condensation happens, which fixes actually work, and the order to do them in.
Why condensation happens
Moist air from cooking, washing and breathing meets a surface below the dew point. It settles on the coldest surfaces, such as single glazing and uninsulated external walls, and mould grows where they stay damp.
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Condensation forms when water vapour in the air hits a surface that is below the dew point temperature. Cooking, showering, drying clothes and even breathing all put water into the air, and government guidance notes that this moisture builds up if it cannot be removed. It then settles on the coldest surfaces: single-glazed windows, uninsulated external walls, corners where two external walls meet, and behind furniture against cold walls.
Mould grows on surfaces that stay damp. Government guidance on damp and mould says damp and mould are a risk to anyone's health, and the risk is greatest for people with an existing health condition such as asthma or another lung condition, people with a weakened immune system, pregnant women, children and young people, older people, and people who are housebound or have limited mobility.
What causes condensation, and how do you fix it?
Cold surfaces, too much moisture and too little heating. Fix ventilation first as it is cheapest and fastest, then insulate, then heat the rooms you use to at least 18°C.
Read the detail
1. Cold surfaces (insulation problem)
The colder a surface, the more likely condensation will form on it. In winter an uninsulated external wall can be several degrees colder on the inside than the room air, so ordinary indoor humidity condenses on it. Government guidance notes that homes that are poorly insulated and heated are more prone to condensation on the inside of external walls and windows.
Fix: Insulate the cold surfaces. Cavity wall insulation raises the temperature of the inside of the wall, and the Energy Saving Trust lists a lower risk of condensation on walls among its benefits. Internal solid wall insulation does the same for older homes with solid walls. Double or triple glazing raises window surface temperature. Even draught-proofing helps by stopping cold air washing over internal surfaces.
2. Too much moisture (ventilation problem)
If moisture produced inside the home cannot escape, humidity builds up. Common sources include drying clothes indoors, cooking without an extractor fan, long showers without ventilation, and portable bottled gas heaters, which give off water vapour as they burn.
Fix: Ventilate. Use extractor fans in kitchens and bathrooms (and actually turn them on). Open trickle vents on windows. Do not block air bricks. Dry clothes outdoors or in a tumble dryer vented to the outside, not on a radiator indoors. If you use a portable bottled gas heater, switch to another form of heating if you can: it adds moisture and needs ventilation to burn safely.
3. Not enough heating (temperature problem)
Cold air holds less moisture than warm air. If you do not heat your home adequately, the air reaches saturation point with less moisture in it, and condensation forms more easily. This is why condensation is worst in unheated bedrooms with the door closed overnight.
Fix: The NHS advises heating the rooms you regularly use, such as the living room and bedroom, to at least 18°C. Keeping the home steadily warm, rather than letting rooms go cold for long spells, also helps prevent condensation. Thermostatic radiator valves let you keep less used rooms cooler without letting them go cold.
The right order
Fix ventilation first (cheapest and fastest). Then insulation (most impactful, may be free through ECO4 until 31 December 2026). Then heating. Do not insulate without addressing ventilation, as a tighter home traps more moisture. Do not just heat more without ventilating, as you are adding energy cost without solving the root cause.
How do you fix it room by room?
Run extractor fans in the bathroom and kitchen, and open bedroom vents or a window each morning. Pull furniture away from external walls, and rule out leaks and other damp before blaming condensation.
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Bathroom
Install or upgrade the extractor fan. It should run during showering and for a while afterwards. If you do not have one, consider a humidistat-controlled fan, which turns on automatically when humidity rises. Keep the bathroom door closed while showering to prevent moisture spreading. Wipe down cold surfaces (tiles, windows) after showering.
Kitchen
Always use the extractor fan when cooking. Cover pans with lids. If you do not have an extractor, open a window while cooking and keep the kitchen door closed. Avoid drying clothes on radiators in the kitchen.
Bedrooms
Keep trickle vents open, or open a window for a short while each morning to clear the moisture that builds up overnight from breathing. Pull furniture a little away from external walls to let air circulate behind it. Do not completely seal bedroom doors overnight if the room has no independent ventilation.
Whole house
A hygrometer shows how humid your rooms are. If humidity stays high day after day and windows stream every morning, you have a ventilation, heating or insulation problem. Rule out leaks and rising or penetrating damp before assuming all moisture is condensation.
A dehumidifier
If you have to dry clothes indoors, or a room stays damp after the fixes above, a dehumidifier pulls water out of the air. It treats condensation only: it does nothing for a leak or for rising or penetrating damp, and it does not replace extractor fans or opening a window. The running cost is modest.
Paid link: we may earn a commission if you order through it. It costs you nothing and does not change any figure on this page.
As an example, Meaco's Arete One 10 litre dehumidifier was £159.99 when we checked on 12 October 2026. Meaco lists it at 151 watts in a room at 20°C and 60% humidity, which is about 4p an hour at the October to December 2026 price cap of 26.32p a kWh. Many makers and shops sell comparable machines, so compare before you buy.
How can you tell it is not condensation?
Damp that is worse in wet weather, or on walls that are not external, may be rising or penetrating damp. A qualified damp surveyor can diagnose it, and tenants should report damp and mould to their landlord in writing.
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Not all damp is condensation. Rising damp (moisture coming up through the floor and walls from the ground) and penetrating damp (rain getting through the walls or roof) require different fixes. If you see damp patches on ground-floor walls that are worse in wet weather, or damp that appears on internal walls that are not external-facing, it may be structural damp rather than condensation. A qualified damp surveyor can diagnose the cause.
If you rent, report damp and mould to your landlord in writing. Since 27 October 2025, Awaab's Law has required social landlords in England to investigate and fix dangerous damp and mould within set time limits.
Landlords can find what the law already requires of them, and what a damp survey costs, in Landlord Planner's Awaab's Law and damp guide.
Does insulation make condensation worse?
No, as long as you keep ventilating. Insulation raises inside surface temperatures, which cuts condensation; it only goes wrong when a tightly sealed home gives the moisture nowhere to go.
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There is a common fear that insulating your home makes condensation worse. This is only true if you insulate without maintaining ventilation. Insulation raises internal surface temperatures, which directly reduces condensation risk. But if you seal a home tightly (with insulation and draught-proofing) and do not provide adequate ventilation, the moisture has nowhere to go.
The solution is simple: insulate and ventilate. Keep trickle vents open. Ensure extractor fans work. If doing a comprehensive retrofit, consider mechanical ventilation with heat recovery (MVHR), which provides fresh air while recovering much of the heat.
For the full insulation picture, read our guides: Is Loft Insulation Worth It?, Cavity Wall Insulation, Solid Wall Insulation, and Draught-Proofing. Use our insulation savings calculator to see the financial return.
Frequently asked questions
What causes condensation in UK homes?
Moist air from cooking, washing, drying clothes and breathing meeting cold surfaces such as windows and poorly insulated walls. Too little ventilation lets the moisture build up, and too little heating leaves surfaces cold. Fix all three: ventilate, heat and insulate.
Will insulation make condensation worse?
Only if you seal the home without keeping it ventilated. Insulation raises the temperature of inside surfaces, which cuts condensation, but keep trickle vents open and extractor fans working, and never block air bricks or vents.
How do I stop mould coming back?
Remove the moisture at source: use extractor fans, dry clothes outside or in a vented dryer, keep trickle vents open, heat the rooms you use to at least 18°C, and insulate cold walls. If mould keeps coming back, look for a leak or other source of damp, and if you rent, tell your landlord in writing.
Data sources
Health risks come from GOV.UK damp and mould guidance and heating advice from the NHS, with ventilation from Approved Document F and insulation benefits from the Energy Saving Trust.
Show every source
Ventilation guidance from Building Regulations Approved Document F. Health risks from Understanding and addressing the health risks of damp and mould in the home (GOV.UK, updated 1 April 2026). Heating advice from the NHS. Awaab's Law from GOV.UK guidance for social housing tenants. Insulation benefits from the Energy Saving Trust.